The present invention relates generally to electric motors and deals more specifically with a high torque variable reluctance motor.
Variable reluctance motors of the type in which a variable reluctance member or rotor is rotatably stepped relative to a stator member by magnetic flux are generally well known in the art. The operation of a typical variable reluctance motor is described in U.S. Pat. No. 3,984,711 issued Oct. 5, 1976 to Kordik. One problem generally associated with variable reluctance motors is that magnetic flux produced in a stator member by the excitation of electrical windings wound around the stator members tends to leak between adjacent and simultaneously excited stator members. This flux leakage between adjacent stator members causes more flux to be effectively used in rotating the rotor to and holding the rotor in a stop or detent position when the motor is used as a stepping motor. Consequently, to overcome the effect of leakage flux more current is required to generate the torque necessary to rotate the rotor and/or to hold the rotor in its stop position. Accordingly, variable reluctance motors generally require a heavier mass for a given torque to dissipate heat generated by the additional power consumption necessary to produce the desired motor torque characteristic.
Variable reluctance motors having a high mass-to-torque ratio are generally undesirable in many applications, such as, for example, robotics wherein the motor is required to lift itself in addition to a robotic arm in which the motor is located. There have been a number of approaches to improve the operational efficiency and increase the torque of variable reluctance motors by confining the magnetic flux produced to a more effective flux path so that the torque of a motor is greater for a given amount of current applied to the windings. Kordik locates permanent magnets between adjacent stator winding pole members to confine the winding created flux to a more effective path.
U.S. Pat. No. 4,475,051 issued Oct. 2, 1984 to Chai attempts to improve the efficiency and increase the torque of a variable reluctance motor by confining the flux generated to a shorter magnetic path. Chai provides recesses in the periphery of the stator assembly which are aligned in the lateral center of the non-winding pole members to restrict the available stator iron through which the magnetic flux passes from the non-winding pole member to the winding pole member. Since a non-winding pole member is shared by two adjacent winding pole members leakage flux from one phase subtracts from the flux produced in the stator of an adjacent phase and which leakage flux results in a reduction in torque.
It is advantageous to use a grain oriented magnetic material for the stator assembly of a variable reluctance motor for increased permeability and lower losses which heretofore has not been possible with variable reluctance motors having a continuous magnetically linked stator assembly.
It is an object of the present invention therefore to provide a variable reluctance motor that has a high torque-to-mass ratio which overcomes the disadvantages associated with known variable reluctance motors.
It is a further object of the present invention to provide a variable reluctance motor that magnetically isolates adjacent stator members from one another to prevent flux leakage between the adjacent stator members.
It is yet a further object of the present invention to provide a variable reluctance motor that is less costly and easily manufactured.